Glycolysis: Key Steps and Mechanisms

Glycolysis

Triose Phosphate Isomerase Reaction

Glycolysis is a central metabolic pathway that converts glucose into pyruvate, generating ATP and NADH. One of the key steps involves the enzyme triose phosphate isomerase (TPI), which catalyzes the interconversion of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P).

• Reaction: DHAP → G3P

• Standard Free Energy Change: $\Delta G^\circ' = +7.5 \text{ kJ/mol}$

• Importance: This step ensures that both products of the aldolase reaction can continue through glycolysis.

• Enzyme: Triose phosphate isomerase (TPI)

Glyceraldehyde 3-Phosphate Dehydrogenase Reaction

From this point, all reactions are doubled per molecule of glucose. The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyzes the oxidation and phosphorylation of G3P to 1,3-bisphosphoglycerate (1,3-BPG).

• Reaction: G3P + Pi + NAD+ → 1,3-bisphosphoglycerate + NADH + H+

• Standard Free Energy Change: $\Delta G^\circ' = +6.3 \text{ kJ/mol}$

• Importance: This step produces NADH, which is used in oxidative phosphorylation.

• Enzyme: Glyceraldehyde 3-phosphate dehydrogenase

Phosphoglycerate Kinase Reaction

The enzyme phosphoglycerate kinase catalyzes the transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.

• Reaction: 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP

• Standard Free Energy Change: $\Delta G^\circ' = -18.5 \text{ kJ/mol}$

• Importance: This is an example of substrate-level phosphorylation, directly generating ATP.

• Enzyme: Phosphoglycerate kinase

Phosphoglycerate Mutase Reaction

The enzyme phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate, preparing the molecule for subsequent steps in glycolysis.

• Reaction: 3-phosphoglycerate → 2-phosphoglycerate

• Importance: This rearrangement is necessary for the eventual formation of phosphoenolpyruvate, a high-energy intermediate.

• Enzyme: Phosphoglycerate mutase

Summary Table: Key Glycolytic Steps

Step	Enzyme	Reaction	$\Delta G^\circ'$ (kJ/mol)
5	Triose phosphate isomerase	DHAP → G3P	+7.5
6	Glyceraldehyde 3-phosphate dehydrogenase	G3P + Pi + NAD+ → 1,3-BPG + NADH + H+	+6.3
7	Phosphoglycerate kinase	1,3-BPG + ADP → 3-PG + ATP	-18.5
8	Phosphoglycerate mutase	3-PG → 2-PG	Not specified

Additional info:

• Glycolysis consists of ten enzyme-catalyzed reactions, divided into two phases: the energy investment phase and the energy payoff phase.

• The steps described above are part of the energy payoff phase, where ATP and NADH are generated.

• Substrate-level phosphorylation is a key mechanism for ATP generation in glycolysis, distinct from oxidative phosphorylation.